Atherosclerosis is the major cause of morbidity and mortality in the Western World. The first recognizable lesion of atherosclerosis, the fatty streak, consists primarily of cholesteryl ester laden macrophages (foam cells) in the arterial intima. The goal of this project is to use murine bone marrow transplantation as a method to investigate the role of the macrophage in atherosclerosis and lipoprotein metabolism. The first specific aim is designed to test the hypothesis that the transfer of bone marrow cells bearing a specific genetic marker into a lethally irradiated host will result, under the appropriate atherogenic conditions, in the delivery of donor monocytes to the arterial intima and in the development of macrophage-derived foam cells of donor origin. The ROSA beta-geo 26 strain of mice which have ubiquitous expression of beta-galactosidase from the Lac Z gene will be used as bone marrow donors in these experiments, providing an easily detectable genetic marker to track the fate of donor macrophages. Once the conditions for repopulation of the host with macrophages of donor origin have been established, we will use this system to investigate the role of apolipoprotein (apo) E secretion by the macrophage in lipoprotein metabolism nd atherosclerosis. The majority of apoE in the plasma lipoproteins is of hepatic origin, and the relative contribution of extrahepatic apoE to the metabolism of the plasma lipoproteins is uncertain. ApoE is a ligand for the LDL receptor and promotes the clearance of several classes of lipoproteins from the plasma. Mice homozygous for the targeted disruption of the apoE gene develop severe hyperlipidemia and spontaneous aortic and coronary atherosclerosis. in the second specific aim, transplantation of bone marrow from mice with the normal apoE gene into apoE deficient mice will be performed to investigate the ability of extrahepatic apoE to contribute to plasma apoE levels and the clearance of plasma lipoproteins. The macrophage is known to secrete large amounts of apolipoprotein E and free cholesterol when exposed to acetylated-LDL in vitro. Foam cell formation can be viewed as an imbalance between cholesterol influx and efflux. ApoE secretion by the macrophage may facilitate cholesterol efflux from the macrophage, thus serving a protective role by preventing foam cell formation. Observations that apoE deficient mice develop spontaneous atherosclerosis, whereas transgenic mice expressing a defective apoE from the liver do not, suggest that macrophage apoE secretion may indeed play a protective role in regard to atherosclerosis susceptibility. In the third specific aim, the role of apo E secretion by the macrophage in promoting cholesterol efflux will be tested in vitro, and bone marrow transplantation experiments will test the hypothesis that apo E secretion by the macrophage influences susceptibility to atherosclerosis in vivo. To test the hypothesis that the inability of the macrophage to secrete apoE results in an increase in the susceptibility to atherosclerosis, bone marrow from apoE deficient mice will be transplanted into C57BL/6 mice. In addition, the transplantation of bone marrow from mice with the normal apoE gene into apo E deficient mice will be tested as a means of promoting regression of atherosclerosis in apoE deficient mice.